The detection of film mode is traditionally used in modern televisions to determine the processes to apply to images received with a view to improving their display quality. Among these processes, it is notably known to convert the frame sequence received into a frame sequence having a higher frequency to suppress the flicker effect that appears on the television screen when the frequency of frames displayed is less than 60 Hz. To do this, in the case of an input frequency at 50 Hz, it is known in the art to double the frame frequency and carry it at 100 Hz. This frequency change consists for example in duplicating the frames of the input sequence, as shown in FIG. 1. In this example, the sequence to be converted, noted as ABC, comprises three frames. The first frame represents image A, the second frame represents image B and the third frame represents image C. The duplication operation consists in transforming this sequence of three frames into a sequence of 6 frames, AABBCC, having a double frequency. The two first frames of the transformed sequence are identical and represent the image A. In the same way, the two following frames represent the image B and the two last frames represent the image C. This duplication operation results in suppressing the flicker effect during the displaying of the new sequence but however creates a judder effect when the displayed images comprise motion.
To eliminate this judder effect, it is known in the prior art to compensate the movement of added frames as shown in FIG. 2. In the example illustrate by this figure, the frame representing image A in the input sequence is converted, in the output sequence, into a frame representing image A and a frame representing a motion compensated image A′. To generate image A′, the motion is estimated between image A and the next image, which is image B, and an intermediary image is created A′ based on the motion estimated between these two images. In the same way, the frame representing image B is converted, in the output sequence, into a frame representing the image B and a frame representing a motion compensated image B′ and the frame representing image C is converted, in the output sequence, into a frame representing the image C and a frame representing a motion compensated image C′. This increase in frequency with motion compensation enables suppressing the flicker effect while conserving a fluidity of motion in the sequence.
This type of processing (augmentation of the frequency with or without motion compensation) is however only valid when the input sequence does not comprise duplicated frames, for example when the sequence is in film format (24 or 25 images per second) or in camera format (50 or 60 images per second).
In particular this processing is not applicable when an input sequence, of film format (24 or 25 images per second) origin, has been transformed into a telecine stream by a format conversion of 2:2 pull-down conversion type or a 3:2 pull-down conversion type with a view to its display on a television (50 or 60 images per second). The 2:2 pull-down conversion is used to pass from 25 images per second (film format) to 50 images per second (PAL/SECAM format). This conversion consists in duplicating each input frame. The 3:2 pull-down conversion is used to pass from 24 images per second (film format) to 60 images per second (NTSC format). Each pair of consecutive input frames is then converted into 5 output frames, the three first output frames being identical to the first input frame of the pair and the two other output frames being identical to the second input frame of the pair. In the remainder of the description, reference is made to a video frames sequence in film mode to designate this type of video frames sequence, that originally was in film mode and was converted with a view to its display on a screen and that now comprises duplicated frames, and video frames sequence in video mode or camera mode designates a sequence of video frames in camera format that does not comprise duplicated frames.
With regard to what has preceded, it is easily understood that, for a sequence in film mode (comprising duplicated frames), the processing applied to reduce the flicker creates a great deal of judder when the frames of the sequence in film mode are simply duplicated (without motion compensation) as there is then duplication of already duplicated frames. This judder problem is also present when added frames are motion compensated because, for some among them, the motion estimation is carried out between two identical frames, one being the duplicate of the other. The motion estimation is then null and the motion compensation of no use.
It is therefore important to detect the mode, film or camera, in the sequence of frames to be processed before applying any processing, such as, for example, that described previously to reduce the flicker effect. It can also be of interest to determine the type of film mode of the sequence (film mode 2:2 or film mode 3:2) and identify in the sequence, the frames corresponding to the frames of the original sequence in film format (24 or 25 images per second) between which a motion estimation and a motion compensation can be viably carried out.
Methods for detection of film mode are known in the prior art. All of these methods are based either on a motion estimation, or on Displaced Frame Difference (DFD) and require the complete storage of at least one video frame. When the detection of film mode is carried out by a programmable circuit of FPGA (Field Programmable Gate Array) type, it is then necessary to add an external memory to the FPGA circuit to store said video frame.